121 Die in Mystery Crash of Airliner

High in the skies above the Aegean Sea, two Greek fighter pilots nudged their F-16s to either side of a Cypriot passenger jet and quickly discerned the disaster that was unfolding.

They saw a cockpit in turmoil. One of the Cypriot pilots sat slumped over the instrument panel. The other was not visible at all. Two other people desperately tried to gain control of the plane. Oxygen masks dangled in the cabin.

Less than an hour later, Helios Airways Flight 522 bound for Athens slammed into the side of a wooded Greek hill just past noon Sunday, killing all 121 people aboard. More than a third of the dead reportedly were children.

The worst airline crash in Greek history is also one of the most mysterious. Incapacitation of both pilots on a commercial jetliner is extremely rare. It suggests multiple malfunctions that happened either too quickly or too subtly for the pilots to be aware of them or to act in time to remedy them, aviation experts said.

Greek and Cypriot officials said Sunday that, though nothing could be ruled out, it appeared unlikely that terrorism was involved. In addition, there were no signs of a hijacking, such as a distress call, the officials said. “We have no indication whatsoever that this was a terrorist act,” Cypriot Transport Minister Harris Thrassou said in a televised news conference at Cyprus’ Larnaca Airport, where the flight originated.

The leading theory about what went wrong on the Boeing 737 was a catastrophic loss of cabin pressure that would have starved the pilots and passengers of oxygen. One pilot had reportedly complained of trouble with the plane’s air conditioning system early in the flight.

At altitudes above 30,000 feet, a sudden decompression could plunge the temperature inside the plane below zero, and those aboard could begin to experience hypoxia and loss of consciousness within 10 to 30 seconds unless they were able to put on emergency oxygen masks, aviation experts said.

The F-16s intercepted the Helios flight at 34,000 feet, officials said.

If cabin pressure loss is to blame, it still does not explain how or why these systems failed, and whether the pilots were alerted by alarms that are standard equipment on jetliners.

Most of the people on board may have been dead or unconscious for the last hour or so that the plane traveled, on automatic pilot, until crashing, a civil aviation official in Athens said. The two who entered the cockpit to attempt to control the plane may have been crew members or passengers, a government official said. Their identities were not clear, nor how long they were able to sustain their efforts.

The decompression scenario was reminiscent of the 1999 crash that killed U.S. golfer Payne Stewart, whose plane flew for nearly five hours after a sudden loss of cabin pressure killed or disabled all on board. Its windows iced over, the aircraft finally plummeted into a field in South Dakota. But that plane was a small Learjet, not a commercial airliner, which has numerous backup systems.

“Although there are precedents for both pilots losing consciousness, for it to happen on a large airliner like a Boeing 737 with all the backup systems they have does seem to be really quite extraordinary,” Kieran Daly, editor of Air Transport Intelligence, said in an interview with CNN. “If the crew had any indication at all of any problem [with air systems] they would immediately be on very robust emergency systems. How that did not happen [with the Helios flight] is extremely strange.”

Sudden loss of cabin pressure would trigger a loud horn and caution lights, said John J. Nance, an air safety analyst and former Air Force and commercial pilot. “You’re not going to miss this, anyway, because your ears are going to start popping.

“There’s a lot here that doesn’t make sense,” Nance said, but added: “Provided the information from the fighter pilot is right, then we have a high, high probability that we had a malfunctioning crew oxygen system, that the crew didn’t know it in time.”

He noted that drop-down oxygen masks provided in the passenger cabin are not powerful enough to sustain consciousness for very long.

It was also unclear why the Helios plane eventually lost altitude and crashed; one possibility was that it ran out of fuel.

In Sunday’s crash, scorched debris, suitcases and bodies were scattered down a hill and across a ravine 25 miles north of Athens. A piece of the tail and a wheel from the landing gear were intact and recognizable. The impact ignited fires in the brush. One report from the scene said some of the bodies were wearing orange oxygen masks.

A man who said his cousin was on board reported receiving a mobile telephone text message from him shortly before the crash, saying the captain had “turned blue” and temperatures in the cabin had plummeted.

“Cousin farewell, we’re freezing,” the passenger messaged, according to Alpha TV, a Greek network. The authenticity of the message could not be verified, but the description would be in keeping with drastic loss of cabin pressure.

By Sunday afternoon, workers at the crash site had recovered the so-called black boxes that contain recordings of flight data and the pilots’ voices, said Akrivos Tsolakis, head of the Greek airline safety commission.

At airports in Larnaca, in Athens and in Prague, Czech Republic, the flight’s final destination, distraught families spent long hours with little information and rising anxiety. Especially at Larnaca, angry relatives criticized Helios for providing information too slowly and scuffled with police when they tried to storm the airline’s offices.

The dead included 48 Greek Cypriot youths on a holiday school trip to Prague, a Helios Airways spokesman said. Most of the passengers and crew members were Greek Cypriot, the spokesman said.

A Boeing Co. spokeswoman said the company had dispatched a team of experts to join the investigation. The plane that went down was manufactured seven years ago and delivered to Helios in April 2004, she said.

Shortly after takeoff Sunday morning, the Helios pilot radioed to ground control in Larnaca that he was having trouble with the air conditioning, Greek officials said. That was the last contact.

The plane crossed into Greek airspace about half an hour later, around 9:30. Athens air traffic controllers began attempting to contact the plane at 10:07 to initiate landing procedures, but received no response. The attempts continued until 10:55, when the lack of communication triggered Greek emergency procedures known as “renegade alert”: The Greek air force scrambled two F-16 fighters to escort the airliner to its landing.

Instead, the F-16 pilots, flanking the Helios aircraft, saw an unconscious co-pilot and no captain in the cockpit, Greek government spokesman Theodoros Roussopoulos said at an Athens news briefing. The fighter pilots noticed that the oxygen masks had deployed inside the cabin and, minutes later, saw two people enter the cockpit to attempt to gain control of the doomed flight, Roussopoulos said.

The presence of these two was especially mysterious, he added, because by all appearances most of the plane’s other occupants were incapacitated.

“When a pilot has no communication with the control tower, the procedure dictates that other planes must accompany and help the plane land,” said Thrassou, the Cypriot transport minister. “Unfortunately, it appeared that the pilot was already dead, as was, possibly, everyone else on the plane.”

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Times staff writer Elizabeth Douglass in Los Angeles contributed to this report.